Sexually transmitted diseases are widespread across the animal kingdom, affecting mammals, birds, reptiles, and even insects. Far from being a uniquely human problem, STDs have shaped animal evolution, behavior, and population dynamics for thousands of years. Some animal STDs closely resemble human infections, caused by bacteria or viruses that target the reproductive tract. Others are stranger than anything found in human medicine, including a contagious cancer in dogs that has been spreading from animal to animal for roughly 11,000 years. The range of these infections, and the damage they cause, is broader than most people expect.
Chlamydia in Koalas
The most widely discussed animal STD is chlamydia in koalas, and for good reason: it threatens the survival of entire populations. Chlamydiosis is a major cause of population decline in koalas, driving both death and infertility at scale.1Scientific Reports. Koala MHCII association with chlamydia infertility remains equivocal: a need for new research approaches The bacterium responsible, Chlamydia pecorum, spreads sexually between adults and can also pass from mother to joey. It causes urinary tract infections, blindness, and severe reproductive damage.
While most attention has focused on female koalas becoming infertile, research shows that males are also affected. Wild koalas with severe clinical signs of chlamydia had significantly greater sperm DNA damage, and in-lab exposure of koala semen to C. pecorum reduced sperm motility and viability over 24 hours.2PubMed. The effect of Chlamydia infection on koala (Phascolarctos cinereus) semen quality Interestingly, some males who had apparently cleared the infection still showed high rates of abnormal sperm, suggesting that the reproductive damage persists even after the bacteria are gone.
The conservation picture is complicated. Modeling studies have found that stable coexistence between chlamydia and koala populations is possible under normal conditions, even in small populations. Extinctions driven by chlamydia alone are predicted only when additional stressors pile on: habitat fragmentation, increased transmission rates, or other factors that push population growth below a critical threshold.3Journal of Applied Ecology. Modelling Chlamydia–koala interactions: coexistence, population dynamics and conservation implications In other words, chlamydia on its own is survivable for koala populations, but combined with human-caused habitat loss, it becomes far more dangerous. Vaccination trials are underway in Australia, though developing an effective vaccine for a wildlife species spread across fragmented habitats remains a significant challenge.
The Oldest STD on Earth
Dogs carry what is arguably the most extraordinary sexually transmitted disease in the animal world: canine transmissible venereal tumor, or CTVT. Unlike most cancers, CTVT is not caused by a virus that triggers tumor growth. It is the tumor itself that spreads. Living cancer cells pass from dog to dog during mating, latch onto the new host’s tissue, and begin growing. The cancer cells are genetically distinct from the host dog, making CTVT essentially a parasitic organism that happens to be made of dog cells.
Genomic analysis has traced CTVT back to a single dog that may have lived about 11,000 years ago. The cancer spawned by that individual persisted, spreading between dogs across millennia. About 500 years ago it dispersed across continents, likely carried by dogs accompanying European sailors and colonists.4PubMed Central. Transmissible dog cancer genome reveals the origin and history of an ancient cell lineage CTVT is now found in dog populations worldwide, particularly in stray and free-roaming dogs in tropical and subtropical regions. The tumors typically appear on the genitals and can grow into large, cauliflower-like masses. Most dogs’ immune systems eventually fight the tumor off, though this can take months, and the tumors sometimes regress on their own. In veterinary practice, CTVT responds well to chemotherapy. Still, the fact that a line of cancer cells has been continuously alive and spreading for over 10,000 years makes it one of the oldest known living cell lineages on the planet.
Cattle and the Economics of Animal STDs
In livestock, sexually transmitted infections carry enormous economic consequences. Cattle are susceptible to several STDs, and because a single bull may breed with dozens or hundreds of cows, a single infected animal can devastate a herd’s reproductive output in one breeding season.
Brucellosis, caused primarily by Brucella abortus in cattle, is one of the most economically damaging. The hallmark sign in females is abortion and retained fetal membranes; in males, it causes orchitis and inflammation of the joints. The disease spreads through direct or indirect contact with infected animals or their discharges, and it is also zoonotic, meaning it can jump to humans who handle infected animals or consume unpasteurized dairy products.5PubMed Central. Bovine Brucellosis: Epidemiology, Public Health Implications, and Status of Brucellosis in Ethiopia While brucellosis has been largely eradicated from dairy herds in many wealthy nations through aggressive vaccination and testing programs, it remains a serious problem in developing countries.
Two other sexually transmitted organisms, Tritrichomonas foetus (a protozoan) and Campylobacter fetus (a bacterium), persistently colonize the lower genital tract of bulls, often without causing obvious symptoms or inflammation in the male. The bull becomes a silent carrier, sexually transmitting the organisms to cows, where they cause reproductive failure including early embryonic death and infertility.6PubMed. Immunity to infections in the lower genital tract of bulls The immune responses of the bull’s genital tract are inefficient at clearing these infections on their own, though systemic vaccination with the relevant antigens can prevent or eliminate them.
Bovine herpesvirus type 1 is another culprit. It causes infectious pustular vulvovaginitis (IPV) in cows, an infection that produces painful ulcers on the vulva and vagina. A recent outbreak in a New Zealand dairy herd found that affected heifers tested positive for bovine herpesvirus type 1 along with other organisms, and biopsies confirmed severe, chronic ulcerative vaginitis.7PubMed. Outbreak of post-parturient infectious pustular vulvovaginitis in a New Zealand dairy herd Mixed infections involving multiple pathogens are common in livestock reproductive disease, making diagnosis and treatment more complicated than a simple one-pathogen, one-disease model would suggest.
Horses and Contagious Equine Metritis
In the horse world, one sexually transmitted disease has been a source of international concern for decades: contagious equine metritis (CEM), caused by the bacterium Taylorella equigenitalis. CEM is highly contagious and spreads through direct or indirect venereal contact. In mares, it causes inflammation of the uterus and short-term infertility, and very rarely, abortion.8PubMed. Contagious equine metritis Stallions, by contrast, show no clinical signs at all. They carry the bacterium on their genitals without ever appearing sick, which is exactly what makes them so effective at spreading the disease to multiple mares during a breeding season.
Both mares and stallions can become long-term carriers. In mares, the bacterium often retreats to hard-to-reach sites like the clitoral fossa and clitoral sinus after the initial infection clears. Experimental inoculation studies found that during the acute phase, the bacterium could be recovered from uterine and cervical samples, but once symptoms resolved it hid in these external sites, detectable in roughly a third of mares. During early estrus, the isolation rate jumped to over half of previously negative mares, only for most to test negative again days later.9American Journal of Veterinary Research. Contagious Equine Metritis: Effect of Intrauterine Inoculation of Contagious Equine Metritis Agent in Pony Mares This intermittent shedding pattern makes surveillance difficult and is a major reason CEM continues to pop up in new outbreaks despite extensive testing protocols in the thoroughbred breeding industry.
Because of the potential economic fallout from a CEM outbreak at a major stud farm, many countries require mandatory testing of stallions and imported mares. The disease is reportable in much of Europe, North America, and Australia, and detection typically triggers quarantine and mandatory treatment of all exposed animals.
Dolphins and Genital Viruses
Wild bottlenose dolphins carry their own sexually transmitted infections, including both herpesviruses and papillomaviruses that target the genital mucosa. Research on a group of Atlantic bottlenose dolphins found that herpesvirus infection was significantly associated with the occurrence of mucosal plaques on the penis or vulva. The virus appears to be endemic in dolphin populations and is likely transmitted through sexual contact.10PubMed. Herpesvirus in bottlenose dolphins (Tursiops truncatus): cultivation, epidemiology, and associated pathology The plaques it causes appear to be benign, similar to how certain human herpesvirus infections cause visible but non-life-threatening lesions.
More concerning is the link between these viruses and genital tumors. At least seven species-specific papillomaviruses and two herpesviruses have been associated with genital tumors in bottlenose dolphins. Some dolphins show evidence of co-infection with both papillomaviruses and herpesviruses, raising questions about whether the viruses interact to drive tumor development, much as certain human papillomaviruses are known to cause cervical cancer in people.11PubMed. Papillomaviruses and herpesviruses: who is who in genital tumor development of free-ranging Atlantic bottlenose dolphins (Tursiops truncatus)? Research on this question is still in early stages, partly because studying the reproductive health of free-ranging marine mammals is logistically difficult.
Primates and the Origins of HIV
One of the most consequential animal STDs in terms of human impact is simian immunodeficiency virus (SIV), the primate relative of HIV. SIV is endemic in several species of African monkeys, where it typically does not cause immune deficiency or obvious disease. These primate hosts appear to have co-evolved with the virus over long periods, developing tolerance rather than resistance. But when SIV crosses into species that have not adapted to it, the consequences are severe. HIV evolved from SIV strains that jumped from primates to humans, and in Asian non-human primates experimentally infected with SIV, the virus actively replicates and causes disease despite cellular restriction factors and immune responses.12PubMed Central. HIV and SIV infection: the role of cellular restriction and immune responses in viral replication and pathogenesis
The contrast between African monkey hosts (where SIV is harmless) and humans or Asian primates (where the related viruses devastate the immune system) is a striking example of how a sexually transmitted pathogen can be completely benign in one host and catastrophic in another. This dynamic is not unique to SIV. Many animal STDs cause mild or no disease in their natural hosts but wreak havoc when they spill over into new species or when environmental stressors compromise the host’s ability to tolerate infection, as with chlamydia in stressed koala populations.
Rabbit Syphilis
Rabbits have their own version of syphilis, caused by a spirochete bacterium closely related to the one that causes syphilis in humans. Rabbit venereal spirochetosis is caused by Treponema paraluisleporidarum and affects both wild and pet rabbits. It spreads sexually and through direct contact between animals.13PubMed Central. Penicillin Treatment Failure in Rabbit Syphilis Due to the Persistence of Treponemes (Treponema paraluisleporidarum Ecovar Cuniculus) in the Focus of Infection The disease produces crusty lesions around the genitals, nose, and lips. It is typically treated with penicillin, though research has shown that treponemes can persist in the focus of infection even after treatment, meaning that apparent clinical cures may not fully eliminate the bacteria. For pet rabbit owners, this is an underrecognized condition. Lesions around the face and genitals are sometimes mistaken for other skin conditions, and the sexual transmission route means it spreads readily in breeding colonies.
Insects and Mites That Spread During Mating
STDs are not limited to vertebrates. Ladybirds (ladybugs) carry a parasitic mite, Coccipolipus hippodamiae, that passes between individuals at a high rate during copulation. In the two-spot ladybird, infected females lay fewer eggs, and the eggs they do lay have drastically reduced viability.14Ecological Entomology. Sexually transmitted disease in a promiscuous insect, Adalia bipunctata The mite can also spread at a low rate between non-mating individuals, but sexual contact is the primary transmission route.
The same mite genus has attracted attention as a potential biological control agent. The invasive harlequin ladybird, a species that has displaced native ladybirds across Europe and North America, is also susceptible. Within two to three weeks of infecting a female, the mite causes a reduction in egg hatch rate and eventually complete sterility.15Biological Control. Potential role of the sexually transmitted mite Coccipolipus hippodamiae in controlling populations of the invasive ladybird Harmonia axyridis Researchers have considered whether the mite could help control harlequin ladybird populations, though whether a sexually transmitted parasite can suppress an invasive species at a landscape scale remains uncertain.
Bed bugs offer yet another variation on sexually transmitted harm. Male bed bugs mate by traumatic insemination, literally piercing the female’s body wall to inject sperm. This unusual reproductive strategy creates a direct route for pathogens to enter the female’s body cavity. Females only mate with recently fed individuals, creating a tight link between feeding and the risk of being infected through mating.16PubMed Central. Female bed bugs (Cimex lectularius L) anticipate the immunological consequences of traumatic insemination via feeding cues Female bed bugs have evolved immune defenses that ramp up in anticipation of mating after a blood meal, a preemptive response to the near-certainty that they will soon be wounded and exposed to microbes during copulation.
Birds That Share Bacteria During Copulation
Even birds deal with sexually transmitted microbes. A field experiment on wild kittiwakes, a type of seagull, demonstrated that bacteria are transferred between mates during copulation and that these bacteria alter the composition and diversity of the female’s cloacal microbial community. When researchers experimentally blocked inseminations, the cloacal communities of mated pairs became increasingly dissimilar over time, and females lost bacteria that had been shared with their male partners.17PubMed Central. Sexually transmitted bacteria affect female cloacal assemblages in a wild bird The female cloaca essentially reverted to its pre-mating microbial state, suggesting that it constitutes a resilient ecosystem that temporarily absorbs sexually transmitted microbes but returns to baseline when mating stops.
Whether these sexually transferred bacteria are harmful, neutral, or in some cases beneficial is an open question. In birds, the cloaca serves as a shared exit for the reproductive, urinary, and digestive tracts, making it a complex microbial environment. Some sexually transmitted bacteria may compete with or displace resident microbes in ways that affect the female’s reproductive success, but research in this area is still in its early stages.
Do Animals Try to Avoid STDs?
Given how common sexually transmitted infections are across the animal kingdom, a natural question is whether animals have evolved behaviors to avoid them. The evidence is mixed. Sexual selection theory predicts that animals should prefer healthy mates and avoid infected ones, and there is some support for this. Infection by parasites can alter an animal’s sexual traits and chemical cues, providing potential signals for a choosy mate to detect.18Animal Behaviour. Impact of infection on mate choice But in practice, the costs of being too choosy, such as missing out on mating opportunities entirely, can outweigh the benefits of avoiding infection.
Experimental work in fish has shown that both the parasite load of the choosing individual and of potential mates affect preference. Highly infected females avoided highly infected males, suggesting that animals can adjust their mate preferences based on their own condition and the apparent condition of available partners.19PubMed Central. Mate choice is affected by parasite infestation rate of the choosing individual as well as of potential mating partners However, a comparative study across primates found no support for several behaviors hypothesized to reduce STD risk. Genital inspection before mating, genital self-grooming after mating, and post-copulatory urination showed no correlation with promiscuity levels across primate species. Even monogamy did not correlate with the life-history traits predicted by the hypothesis that it evolved as an STD-avoidance strategy.20Animal Behaviour. Behavioural defences against sexually transmitted diseases in primates
The picture that emerges is that while individual animals sometimes show infection-avoidance behavior in controlled settings, the broad evolutionary patterns do not show clean, universal anti-STD behavioral strategies. STDs are, in a sense, an unavoidable cost of sexual reproduction for many species, kept in check not by behavioral avoidance but by immune defenses, tolerance mechanisms, and the population-level dynamics of host and pathogen co-evolution.
Why Animal STDs Matter for Human Medicine
Animal STDs are not just curiosities for wildlife enthusiasts. They serve as natural laboratories for understanding sexually transmitted infections in humans. Animal models of chlamydia, gonorrhea, and herpes simplex virus type 2 have been instrumental in studying how these infections establish themselves, how the immune system responds, and how vaccines might work.6PubMed. Immunity to infections in the lower genital tract of bulls The discovery that systemic vaccination in bulls can produce protective antibodies in genital secretions, for instance, has informed thinking about mucosal vaccine strategies for humans.
The zoonotic dimension adds another layer of relevance. Brucellosis jumps readily from cattle to humans. SIV’s cross-species leap gave rise to HIV. The papillomaviruses and herpesviruses found in dolphin genital tumors belong to the same viral families that cause cervical cancer and genital herpes in people, though the specific viruses are species-adapted and do not cross between dolphins and humans. Studying how these pathogens evolved to exploit the genital niche in different species helps researchers understand the evolutionary pressures that drive STD virulence, transmission, and host adaptation more broadly. The deep parallel between animal and human STDs is a reminder that sexually transmitted infections are not a byproduct of human behavior, but a fundamental feature of sexual reproduction across the tree of life.